义 在复合句中充当主语成分的句子叫做主语从句. 第一部分 常规主语从句，即句子在复合句中充当一个主语 主语从句的时态：不受主句的时态影响和限制。 （1）That he finished writing the composition in such a short time surprised us all. （2）Whether we will go for an outing tomorrow remains unknown. （3）Who will be our monitor hasn't been decided yet. （4） Whom we must study for is a question of great importance. （5）What caused the accident remains unknown. （6）Whatever you did is right. （7）Who the watch belongs to was lost is unknown. （8）What we need is time. （9）What we need are good doctors. 小结: （1)引导主语从句连词有that,whether,who,what,whatever等 （2)连词位于句首不能省略 （3)主语从句大多数情况下视为第三人称单数,但也有例外,如例（9） 第二部分 为了防止句子头重脚轻，通常把形式主语it放在主语位置，真正主语搁置于句末 （1）It is certain that he will win the match. （2）It is true that he has made a very important discovery in chemistry. （3）It is very likely that they will hold a meeting. （4）It is strange that he should do that. （5）It is important that we all should attend the meeting. （6）It is strange that the man should have stuck to his silly ideas. （7）It is a pity that we won't be able to go to the south to spend our summer vacation. （8）It is still a mystery what caused the accident. （9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai) （10）It is known to all that the gun powder was first invented by the Chinese. （11）It is suggested that the work should be done with great care. （12）It seems that he has seen the film.(=He seems to have seen the film) （13）It happened that the two cheats were there. (=The two cheats happened to be there) 小结: （1)以that 引出的主语从句,常以形式主语it引导. It is +形容词/名词/某些动词ed + that 从句. （2) 在有些that从句中要用虚拟语气 (should+do/should+have done)例句4,5,6,11.编辑本段主语从句的用法 主语从句与宾语从句两者的用法和在句中的作用是不同的，下面从它们的各自用法来介绍。 一．主语从句 主语从句 定义:在句子中担当主语的是一个从句,这个从句就叫做主语从句。 主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。 1. It 作形式主语和it引导强调句的比较 ①主语从句常用it作形式主语，一般常用句型为：It is+{名词/形容词/过去分词}+主语从句例如： It is still a question whether she will come or not. It is strange that you should like him. It is still unknown which team will win the match. 另外，还有一些比较多见的结构： It turned out that……； It has been proved that……; It happened/occurred that……； It is well-known that……等等 ②而强调句则不同，它的结构是：It+be+被强调部分+that+从句 强调主语：It is the times that produce their heroes.时势造英雄 强调宾语：It is English that Prof.Lin teaches us. 强调状语：It was in shanghai that I saw the film. 判断是否是强调句有一个方法，就是将that以后的“句子的其余部分”拿出来单独看，看有没有缺成分，缺的是什么成分，再把谓语动词后面that前面的那部分，带到你认为缺成分的地方，如果放进去是一句完整的句子了，那就说明是强调句。 2. 用it 作形式主语的结构 (1) It is +名词+that从句 It is a fact that … 事实是… It is an honor that …非常荣幸 It is common knowledge that …是常识 (2) it is +形容词+that从句 It is natural that… 很自然… It is strange that… 奇怪的是… (3) it +不及物动词+that从句 It seems that… 似乎… It happened that… 碰巧… (4) it is+过去分词+that从句 It is reported that… 据报道… It has been proved that… 已证实… 3. 主语从句不可位于句首的五种情况 (1) if 引导的主语从句不可居于复合句句首。 (2) It is said , (reported) …结构中的主语从句不可提前。例如： It is said that Presid

　　如果选that的话意味着后面那句话是陈述句，没有疑问含义，这句话的意思就变成：那个男人不尽快报警是不清楚的，而其实句子想要表达的是那个男人没有及时报警的原因还不清楚，要用why来表示原因。

　　这名男子没有早点报告这次事故的原因根本没有搞清楚。 It在句中是形式主语，真正的主语是why引导的主语从句。 选择that语法上是正确的，但是从语义上不完整。你可以自己翻译一下： 那个男人没有立即报告那起事故，这个事情不得而知。 这个语义是不完整的。应该是“为什么不报告”